Ventricular suction, the phenomenon whereby the ventricle assists its own filling by contracting to below its equilibrium volume and thereby generating restoring forces, has been recognized for many years. Its physiologic importance and mechanism remain uncertain, however, due to the difficulty of studying suction in the absence of the confounding influence of other determinants of filling, especially those related to the properties of the atrium. Recent studies in which a prosthetic mitral valve was used to clamp left ventricular (LV) volume at the end- systolic (ES) value suggest that suction is a major component of filling under physiologic conditions. To study this phenomenon in the Intact LV, we have designed a servo motor system which allows abrupt clamping of left atrial pressure (LAP) at a constant value during a single diastole. By clamping LAP below LV minimum pressure, a non- filling diastole (identical to an ES volume clamp) can be produced which in turn allows delineation of the positive and negative, fully- relaxed pressure-volume relation (PVR) of the LV. The position of the negative PVR Is a measure of the magnitude of restoring forces. In addition, the system can be used to produce zero source pressure conditions for filling, in which LAP = LV external pressure. With a zero source pressure, filling can only occur via suction. This allows study of suction in the absence of the effects of other determinants of filling. In this project, to be performed in open-chest pericardiectomlzed dogs, we propose to exploit this unique approach to accomplish four specific aims: 1) determine the Importance of suction as a mechanism of filling over a wide range of preload, afterload, heart rate and contractility; 2) delineate the features of normal regional contraction responsible for generation of restoring forces and how they are modified by alterations in regional function; 3) relate restoring forces/suction to LV "twist"; and 4) determine the relationship between LV restoring forces and right ventricular performance. These studies should provide improved understanding of an important aspect of LV filling.